1. Field of the Invention
This invention relates to the field of surgery, and more particularly to a stylet and methods of use for facilitating the performance of surgical procedures involving the placement of retrograde cardioplegia catheters.
2. Background Art
Since the early days of cardiac surgery, it has been recognized that in order to provide the optimum surgical conditions when operating on the heart, it is necessary to interrupt the normal operation of the heart. For obvious reasons, an arrested, flaccid heart is preferred during a cardiac surgical procedure over a beating heart with blood flowing through it. Thus, in order to be able to efficiently perform cardiac surgery, it is often necessary to use cardiopulmonary-bypass techniques and to isolate the heart from its life-giving blood supply.
It has been found that many deaths occurring after cardiac surgery are due to acute cardiac failure. At first, it was believed that the heart was simply beyond repair and that the operation had failed to correct the problem. Later, it was discovered that many of these postoperative deaths were due to new, and often extensive, perioperative (during or within 24 hours after the surgical procedure) myocardial necrosis (death of the heart tissue). Furthermore, many patients who survived were found to have suffered myocardial necrosis to a significant degree, thereby resulting in low cardiac blood output.
It is now known that myocardial necrosis occurs because the energy supply or reserve of the cardiac muscle cells is inadequate to supply the needs of the heart. The availability of oxygen dramatically affects the cell's ability to satisfy these energy requirements. For example, anaerobic metabolism of glucose produces two (2) moles of adenosine triphosphate ("ATP") per mole of glucose (as well as harmful acid metabolites), whereas aerobic metabolism of glucose produces thirty-six (36) moles of ATP per mole of glucose. Therefore, one of the primary goals of myocardial preservation techniques during surgery is to reduce myocardial oxygen consumption.
Myocardial oxygen consumption is significantly reduced by stopping the electromechanical work of the heart. The oxygen demands of the beating empty heart at 37.degree. C. are four to five times those of the arrested heart (i.e., 4-5 ml/100-gm/min compared with 1 ml/100-gm/min). Buckberg, G. D., "Strategies and Logic of Cardioplegic Delivery to Prevent, Avoid, and Reverse Ischemic and Reperfusion Damage," 93 The Journal of Thoracic and Cardiovascular Surgery, 127, 136 (January 1987) (hereinafter referred to as: Buckberg, "Strategies and Logic of Cardioplegic Delivery").
One technique developed for stopping the electromechanical activity of the heart involves halting the heart by infusing high potassium cardioplegic solution. Cardioplegia, which literally means "heart stop," may be administered in an antegrade manner (through arteries in the normal direction of blood flow), in a retrograde manner (through veins opposite the normal blood flow direction), or in a combination of retrograde and antegrade administration. Cardioplegic solutions, typically containing potassium, magnesium procaine, or a hypocalcemic solution, stop the heart by depolarizing cell membranes. Blood is then redirected around the heart through a series of venous return tubes and catheters.
To gain access to the heart so that cardioplegic solution may be infused, early techniques utilized a surgical procedure requiring dissection of both cavae, the placing of tourniquets around the cavae, and the performance to two artiotomies. This allowed the positioning in the coronary sinus of a cardioplegia catheter through an incision in the right atrium. The cardioplegia catheter was held in place by inflating a retention balloon which surrounded the distal tip of the cardioplegia catheter. When inflated, the balloon wedged against the walls of the coronary sinus. Cardioplegic solution was then infused into the heart until it stopped beating. This procedure was repeated several times until the surgery was completed.
One serious disadvantages to this procedure was that the diseased and damaged heart was exposed to even further trauma by the need to dissect both cavae for visual verification of the cardioplegia catheter placement.
To overcome the need for cavae dissection prior to the administration of cardioplegic solution, stylets were developed which could be placed within a cardioplegia catheter to manually guide the catheter in place in the coronary sinus. These stylets were inserted through a very small slit instead of the large wound required for visual verification. A stylet is a thin rigid rod with a rounded tip to prevent puncturing of the cardioplegia catheter during installation. A stylet is inserted into an empty lumen or passageway that extends along most of the length of the catheter. The large lumen the stylet during installation of the catheter. The use of a rigid stylet gives the normally soft and pliable catheter enough rigidity to be manipulated within the heart from a position outside of the heart.
After insertion of the catheter into the heart, the catheter is then simply secured in place with a purse string suture, and the stylet is withdrawn from the catheter. Cardioplegic solution is then infused into the heart halting its pumping action. Once securely positioned, the catheter remains in place for the duration of the operation.
Although the use of a stylet decreased the trauma to the heart by obviating the need for cavae dissection, its use posed other problems. The passage of the catheter through a small incision blocked the visual placement of the catheter in the coronary sinus. Surgeons using a stylet were now required to guess the location of the catheter by observing the reaction of the heart and the pressure of the cardioplegic solution passing through the catheter.
In addition, stylets have only small hubs, left over from the manufacturing of the stylet, on which to grip the stylet during placement. This small handle limits the tactile feedback necessary to determine the location of the catheter within the heart.
One additional problem encountered when using a stylet occurs as the catheter is being positioned with the stylet in the coronary sinus of the heart. Since the heart is not dissected, pressure may remain within the heart which tends to fill the retention balloon of the cardioplegia catheter prior to final placement. Most retention balloons are designed to be inflated by the flow of cardioplegic solution through the balloon. After passage through the balloon, the cardioplegic solution flows into the heart. Pressure from the heart, however, may cause pressurized blood to flow backwards through the tip of the catheter and fill the balloon from the heart side. Since the surgeon cannot see the position or condition of the balloon, the premature back-filling may mislead the surgeon as to the location of the catheter within the heart or may inextricably wedge the balloon in an undesirable location. Mislocation of the catheter can lead to hemorrhage and other deleterious effects.